The photodetection device for detecting an amount of incident light includes a photodiode that generates a charge according to an amount of incident light and a signal processing device that outputs an electrical signal having a value according to the amount of the charge generated in this photodiode. As an example of such a photodetection device, a photodetection device disclosed in Patent Literature 1 is known. The photodetection device disclosed in this literature has an A/D conversion function, and can output a digital value according to the amount of incident light.
The photodetection device may be used as, for example, a detecting unit of an X-ray CT apparatus, and a large number of photodiodes may be arranged in an array and covered with a scintillator. Scintillation light is generated when X-rays are made incident on the scintillator, and when the scintillator light is made incident on any of the photodiodes, a charge is generated in the photodiode, and the charge is converted to an electrical signal by the signal processing device.